In NTSC and PAL television systems it is common practice to combine video luminance ("Y") and chrominance ("C") signal components to form a single composite video ("CV") signal for transmission. After transmission, it is usually necessary to separate the composite video signal into its component parts, luminance and chrominance, for further signal processing.
An elementary method of separation comprises low-pass filtering a composite video signal to obtain the luminance component and bandpass or high-pass filtering the composite signal to obtain the chrominance component. This approach, while having the virtue of simplicity, has the disadvantage of not being an efficient form of separation since the composite signal is transmitted with the chrominance component spectrally interleaved with the luminance component. Much better separation may be realized by means of comb filtering techniques. Such filters exhibit periodic band-pass and band reject characteristics which can efficiently separate the components of a composite signal. One method of applying comb filtering comprises comb filtering the composite video signal to obtain a separated chrominance component and then subtracting the separated chrominance component from the composite video signal to obtain the separated luminance component.
As an example, in a digital composite video separator the composite signal is first sampled and converted to digital form. The samples are then delayed in a memory and the delayed samples are subtractively combined with the non-delayed samples to produce an output signal which comprises two components. One of the components comprises a "vertical detail" component which represents luminance detail in a vertical direction and occupies frequencies below the chrominance signal band. The other portion is the desired chrominance signal. To obtain only the chrominance signal it is necessary to filter the combed signal to exclude the vertical detail signal component. Once this is done, the remaining separated combed chrominance component may be subtractively combined with the composite video signal to obtain the luminance component. Since the luminance component was formed by subtraction of the separated chrominance component, the luminance component will be un-combed below the chrominance band and combed within the chrominance band at locations of the chrominance signal spectra.